The 73G is Alaska's ideal "long-haul" aircraft in terms of operational efficiency. The aircraft can fly about 700 miles further than the 739, and is configured with 52 fewer seats than the 73G. When you take into consideration that Alaska is a new player in the long-haul market and that the long-haul routes AS serves are not especially high volume, you can see why the capacity of the 73G is perfect.
For Alaska, the 737-900 is better suited to its medium distance, high volume routes such as SEA-ANC and SEA-LAX. Alaska's goal with the 737-900 was to help compensate for inflated demand at peak times of the day by adding more seats, rather than more flights. This could be especially useful at an airport like LAX, PHX, SAN or SFO, where Alaska has limited gate space.
The 737-900 is well suited to the ANC-SEA route because of its cargo capacity (which amounts to even more than that of the 757-200, if you can believe it). So during the peak season, you can not only carry a lot of tourists, but also a lot of fish in the cargo hold.